Frequency synthesizer circuits are commonly used as accurate frequency sources in a wide variety of electronic devices, including many types of radio communication systems. A frequency synthesizer generally operates by generating one or more output signals having a frequency related to a frequency of an input reference signal. Typically, such circuits are constructed using a single quartz-controlled reference oscillator combined with a phase-locked loop (PLL) to provide a multitude of output frequencies traceable to the highly stable reference from the oscillator.
A basic PLL frequency synthesizer general operates by using a comparator to compare an input reference signal and a feedback signal to produce an error signal. This error signal is typically low pass filtered and used to drive a voltage-controlled oscillator (VCO) which creates the output signal at an output frequency. The output signal is also directed through a feedback loop. The feedback loop typically includes a programmable frequency divider for controlling the output of the synthesizer and to generate the feedback signal. In operation, if the output frequency drifts, the magnitude of the error signal will increase. The change in the error signal will then drive the frequency in the opposite direction in order to reduce the amount of error. As a result, the output frequency of the output signal is effectively locked to the input reference signal.
In conventional radio communications systems, a basic PLL frequency synthesizer generally cannot operate over a very wide range of frequencies, because the comparators generally have a limited bandwidth and can suffer from aliasing problems. These problems can lead to false locking situations or even the inability to lock. Additionally, it is generally difficult to make a high frequency VCO that reliably operates over a wide range of frequencies. This is due to several factors, but the primary restriction is that the amount of phase noise increases as the frequency range of the VCO is increased. As a result, PLL frequency synthesizers are typically limited for use for operating in a single frequency band to avoid introduction of noise.